1. Field of the Invention
The present invention relates to a digital filter device capable of changing the gain.
2. Background Art
Conventionally, a digital filter is applied to an equalizer or the like. If a digital sound signal passes through the equalizer, the digital sound signal is subjected to an arithmetic operation according to a filter characteristic determined by a filter coefficient of the digital filter.
In order to change a filter characteristic, a filter coefficient has to be changed. When changing the filter coefficient, depending on the level of the digital sound signal or the level of change of the gain, a sharp noise, such as pop noise or step noise, can appear in the output digital sound signal.
According to a conventional technique for reducing such noise, for example, a filter coefficient corresponding to a parameter that determines the frequency characteristic of the filter is reduced, thereby reducing the noise.
However, data is changed while performing the normal arithmetic operation on the input digital sound signal, and therefore, it is difficult to prevent noise from occurring at the point of change. In addition, for a system in which the data is changed within a wide range, a storage region capable of storing an enormous amount of data is wastefully needed.
According to another conventional technique, there is proposed a gain changing circuit that is triggered to change the gain of a digital sound signal when a waveform of an analog sound signal becomes the zero level. This gain changing circuit prevents noise from occurring when changing the gain characteristic of the output signal (refer to Japanese Patent Laid-Open Publication No. 9-130245, for example).
However, this conventional technique cannot compensate for a change of a filter characteristic that occurs when changing the gain characteristic of the output signal.